Abstract

A method for dealing with three of the major problems in hybrid rule-based expertsystems is proposed. They are; the problem of managing the complexity of theknowledge base, the problem of graceful degradation and the problem of the logics usedto manipulate the rules, which do not fit easily with human expertise. An experimentalexpert system, Aristotle, is developed in which modules are used as a tool to reducecomplexity. Modules group related rules together into components, which interact withthe outside world through parameters. The module also provides a suitably sized objecton which extra (partial) knowledge can be attached. It is proposed to use partialknowledge to reason, at a general level, about components and groups of components,giving the appearance of a knowledge base, which degrades gracefully. A five-valuelogic is introduced with the extra logic values irrelevant, do-not-know and unknown.This logic is able to reason about unusual situations without having to explicitly checkfor them. This thesis demonstrates, that modules, partial rules and the five-value logic,can overcome, to an extent, the problems mentioned earlier.

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